Process for obtaining bright zinc coating



Patented Dec. 7, 1937 d PATE T OFFICE PROCESS FOR. OBTAINING BRIGHT ZINC COATING John A. Henricks, Chicago, Ill., assignor to Udy- }inte Company, Detroit, Mich, a corporation of No Drawing. Application May 2, ms,

. Serial No. 77,528

13 Claims. -(Cl. 20418) This invention relats to electrodeposition of zinc, particularly from an alkaline bath which has, or can be given, a bright'lustrous appearance. I It has been found previously that good zinc plates can be obtained only when the purity of' g 16 B. Stockton, and dilute nitric acid, United States Patent 1,816,837 to Carl L. Ganser. However, the plates obtained in this manner are not as lustrous as is sometimes desired. Thus. one object of this invention is to produce zinc plates which have 20 a mirror-like lustre.

It has been found that plain zinc cyanide solutions are rather sensitive to impurities. Thus 1 another object of this invention is to .produce zinc platins solutions which will tolerate higher 25 amounts of impurities, particularly lead and cadmium, and still produce bright or bright dippable plates.

I have found that these desirable results can be had by the addition to the zinc plating baths '30 of small amounts of thiourea formaldehyde resins Time thiourea formaldehyde resins can be manufactured according to any of the processes heretofore known in the art or according to the 35 fogging preferred methods which I have origi- 40"gramsof thiourea, 75ml. of a technical formaldehyde water solution containing 37% formaldehyde,-and 1 of caustic soda are iomixed. heated and be down until anorange colored syrup' is obtained. This thiourea formaldehyde resin when solubiliaed as described below and added to alkalineflnc plating baths was quite eil'ective in reducing the grain size of the 45 plated zinc and produceda fair butnot a high lustre A second formula for making thiourea formaldehyde resin is as follows: a 40 grams oi thiourea, 75 ml; of technical 37% 50 water solution formaldehyde, and 1 gram of tartaricacidwereheatedtoboilingandthe reaction allowedtocontinue for commons. Thisgave a whitepreclpltationorrfinwhichwasusedas a brlshtenerin line cyanide half pound ofv tartaric acid.

saveremltscomparabletothereslnby the first above-mentioned formula. This resin gave good grain refinement and some lustre when added to the standard zinc cyanide plating bath in amounts ranging from .5 to 1.5 grams per liter. This resin was also solubilized as described below.

Purer chemicals may be usedin the condensation but the ordinary technical grades are fully satisfactory and cheaper.

The function of the caustic soda and tartaric acid in the above reactions is that of a. catalyst. Any alkaline salt can be, used for this purpose such as caustic potash, sodium and potassium carbonate, sodium and potassium sulphocyanate, cyanide of sodium or potassium, or sodium or potassium acetate, or other alkali salts of organic acids. The amounts required are pro portional to the molecular weight of the salt.

One can also use an acid catalyst such as one- Proportional amounts-of citric, oxalic, acetic. furoic, may also be used with equal success. also small amounts of mineral acids.

Both of the above resins were solubilized by the addition of cresol. The cresol preferably is added to the original mixture of thiourea and'form-. 5 aldehyde before they are 'reacted in the presence of a catalyst. The cresol solubilizer not only makes the resin more easily dispersable in the plating solution, but it also prevents the precipitation of insoluble zinc compounds with the resin.

i In its absence, the resin is slowly removed from the bath and gradually loses its eifect. Among other similar compounds which have proven satisfactory are phenol and phenol-sulphonic acids,

quinol, naphthol and naphthol sulphonic acids. 5

The resin may also be solubilized in other manners. One gram of the resin may be dissolved in 10 ml. of 66 Be. sulphuric acid at room temperature. This mixture is poured into a solution containing25- grams of sodium carbonate in 150 ml. of water, which neutralizes the excess sulphin'ic acid. This gives a. flocculent precipitate of a sulphonated resin which dissolves to the extent of about. 85% on boiling. Sulphonated resin gives good plate, although the lustre has been lessened slightly by the sulphonation.

Ten grams of the resin may be dissolved in 25 ml. of a solution containing 5 grams'of urea, and the mixture boiled for 20 minutes. This gives a clear, yellow syrup which is completely dispersable in the zinc bath and which gives a. lustrous deposit.

A third formula is as follows:

' 29 grams of thiourea, 50 grams urea, 53 ml. of

37% technical water solution oi formaldehyde,

and 1 gram of tartaric acid was mixed, heated and boiled down until a viscous syrup was obtained. In this formula the urea acts as a solubilizer or modifier and thus it is not necessary to solubilize this resin further according to the manner in which the above mentioned resins were solubilized as described above. This resin when used as a brightener in a zinc cyanide plating bath gave good grain refinement and some lustre at amounts ranging from .5 to 1.5 grams per liter.

In testing the brightening efiect of these resins, I used a zinc solution containing 4.5 oz./gal. of zinc, 9 oz./gal. of total cyanide, expressed as sodium cyanide, and 10 oz./gal. of total caustic soda. This bath had been prepared from a pharmaceutical grade of zinc oxide and good technical grades of sodium cyanide and caustic soda. The anodes employed were at least 99.97% pure zinc. Less pure grades may be employed if the solution is purified subsequent to make up and if the anodes are alloyed with mercury and such low current densities are applied that the mercury only amalgamates with the impurities and drops to the bottom of the tank and does not 'go into solution.

In the above bath we find that the optimum concentration of the standard resin is between .5 and 1.0 g./l. Bright plates are obtained at 10 amp/sq. ft. and very brilliant plates at higher current densities. At lower concentrations of brightener the plates tend to be streaky at low current densities; and at higher current densities the plates are not very bright, although there is a great deal of grain refinement. At higher concentrations only the low current density plates are bright while the plates obtained at higher current densities are ribbed or burned on the edges.

In general the solubility of thiourea formaldehyde resin in the. plating bath is not critical. However, the brightening eifect of the resin decreases with increasing solubility and vice versa the brightening effect ofthe resin increases with decreasing solubility of the resin. In other words, the greater the solubility of theresin the less brightening efiect and the lesser the solubility of the resin the greater the brightening effect.

The use of larger amounts of solubilizer in pre- "paring the resin makes it necessary to use higher concentration of resin in the bath. Slightly different optimum concentrations are found when different catalysts or solubilizers are employed, but generally the optimum concentrations vary between .1 and 2.5 g./l.

As stated before, the low current density plates are generally not as bright as the high current density plates, consequently the brightness of a deeply recessed part varies from the high current density points to the low current density points. If the concentration of brightener is too low, the lustre may not always be as high as desired and the plate may have a slightly brownish cast.

-These difllculties are overcome by the use of bright dips. as mentioned above, whereby the lustre is equalized or increased.

when the optimum amount of resin has been addedto the zinc bath, its tolerance to copper is increased about six'-times;'to cadmium more than three times; to lead at least 25 times.

In general the required concentration of brightener increases with increasing metal and caustic soda content in the bath and decreases with decreasing sodium cyanide content.

I claim:

1. The method of electrodepositing zinc, comprising electrodepositing said metal from a cyanide plating solution containing a thiourea formaldehyde resin which is soluble in the cyanide plating solution.

2. The method of electrodepositing zinc, comanide plating solution containing a thiourea formaldehyde resin formed by reacting thiourea and formaldehyde in the presence of a catalyst in the form of an acid.

6. The method of electrodepositing zinc, comprising electrodepositing said metal from 2, cyanide plating solution containing a thiourea f ormaldehyde resin formed by reacting thiourea and formaldehyde in the presence of a catalyst in the form of an alkaline salt.

7. The method of electrodepositing zinc, comprising electrodepositing the said metal from a plating bath consisting of a relatively pure cyanide solution and containing a thiourea formaldehyde resin which ls soluble in the cyanide plating solution.

8. The method of electrodepositing zinc, comprising electrodepositing said metal from a relatively pure cyanide plating solution containing a thiourea formaldehyde resin which is soluble in the plating bath and in the presence of a pure anode of the metal which is to be deposited.

9. A plating bath for electrodepositing zinc comprising a cyanide solution of said metal and a thioureau formaldehyde resin soluble in the said cyanide solution.

10. A plating bath for electrodepositing zinc comprising a cyanide solution of said metal and a solubiiized thiourea formaldehyde resin.

11. A composition of matter comprising a mechanical mixture of zinc oxide, sodium cyanide, caustic soda and thiourea formaldehyde resin which mixture is water soluble.

12. A composition of matter comprising a mechanical mixture of zinc cyanide, caustic soda, sodium cyanide and thiourea formaldehyde resin which mixture is water soluble.

13. A composition of matter comprising a mechanical mixture of a zinc salt, caustic soda, sodium cyanide and thiourea formaldehyde resin which mixture is water soluble.

JOHN A. HENRICKS. 

